8.7. Results and Discussion
8.7.3. Lipid biomarker: phytoplankton productivity and community structure
The Corg value ranges from 0.08 to 0.61% Fig. 8-2c, with the highest values, noted at the middle Pleistocene (Matuyama-Brunhes boundary).
Brassicasterol, dinosterol and alkenone concentration ranges from 0.1 to 3.4 μg/g, 0.1 to 4.9 μg/g and 0.32 to 3.9 μg/g respectively. Profile of lipid biomarkers (alkenone, brassicasterol, and dinosterol) showed different profile, with brassicasterol and dinosterol showing high concentration during
the mid- Pleistocene (Matuyama-Brunhes boundary) in contrast to alkenone concentration (Fig. 8-8a, Fig. 8-8b, Fig. 8-8c). C28 1, 14 diol, C30 1, 14 diol and C30 1, 15 diol were the most dominant alkyl diol identified with values ranging from 0.34-1.9 (μg/g), 0.15-1.5 (μg/g) and 0.58-3.1(μg/g) respectively.
The highest values occurred during the mid- Pleistocene for C28 1, 14 diol and C30 1, 14 diol while C30 1, 15 diol recorded the highest values during the early Pleistocene. In other to study the productivity and upwelling intensity of the central equatorial Pacific we utilized a diol index proposed by Rampen et al., 2008. The diol index record from PC 932 showed a distinctive trend; values were low in the early Pleistocene and increased at the Matuyama-Brunhes boundary (Fig. 8-9d). High values of this index indicate increased productivity and upwelling intensity and vice versa.
Fig. 8-5. Comparison of the down-core profiles of (a) linear sedimentation rate, (b) calcium carbonate percentage, (c) calcium carbonate accumulation rate, and (d) organic carbon accumulation rate.
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Fig. 8-6. Composition of oxygen and carbon isotope record from benthic foraminifer in PC 932.
The Corg values are said to be influenced by the initial productivity in the marine ecosystem, with higher values indicating high biological productivity. It is generally known that the production of organic matter favors light isotope, observed from the δ13C, the low value during the mid- Pleistocene (Matuyama-Brunhes boundary) could have resulted from the increased biological productivity. However a question could arise if the Corg values literally indicate productivity during this period as organic matter degradation, preservation, sedimentation rate and flux of organic carbon through the water column could also affect the shape of Corg records. The lack of coherence between Corg, Corg MAR, and sedimentation rate implies that Corg deposition is primarily controlled in the core by the Corg rain (productivity) from the euphotic zone. This is also in conjunction with the coherence of lipid biomarkers MAR (dinosterol and brassicasterol)
with Corg, indicating higher biological productivity in the middle Pleistocene (Matuyama-Brunhes boundary) (Fig. 8-2c, Fig. 8-8a, Fig. 8-8b).
Surprisingly biogenic opal which has played a major role in paleo-productivity studies showed a contradicting profile with lipid biomarker (brassicasterol), a biomarker for diatom. Biogenic opal production accumulation in sediment depends on silicate and iron (Dugdale and Wilkerson, 1998) and under iron-limited condition; diatoms grow thicker shells due to silicate uptake resulting to a stronger frustules (Wilken et al., 2011) which sink faster and thus better preserved (Wefer et al., 1999). As a consequence, the downward profile of biogenic opal does not reflect productivity but opal accumulation due to the change in the supply of iron into the surface water (Fig. 8-10). Revealed by dinosterol, brassicasterol, and the diol index, the enhanced marine productivity between the Matuyama-Brunhes boundary could be linked to the strength of the southeast trades that drives upwelling in the equatorial Pacific influenced by the latitudinal temperature gradients in the Southern Hemisphere. The result of Ba/Ti, Al/Ti and P/Ti from Murry et al (2000), indicating a pattern of greater export productivity during 800-560 kyr, overlapping with the mid-Pleistocene transition, a period of change in deep water circulation is a confirmation of the lipid biomarker from our study.
Diatoms typically have larger cells, faster division rates, and simpler test structures compared to coccolithophores, resulting in a dominance of the phytoplankton population in nutrient rich, low stratified waters during phytoplankton bloom (Barber and Hiscock, 2006). On the other hand, the relative contribution of coccolithophorid (alkenone) to phytoplankton assemblages is greatest in well-stratified oligotrophic waters. As stated in section 2.1 coccolithophores tend to surpass diatoms and dinoflagellates under a low nutrient and high temperature while diatoms and dinoflagellates bloom in high nutrient and cold environment (Chen et al., 2007). Thus oceanographic condition during the early Pleistocene favored coccolithophorid (alkenone), suggesting that the supply of nutrients to the surface was reduced during the early Pleistocene. Furthermore, from the occurrence of long-chain diols from PC 932 (Fig. 8-9a, Fig. 8-9b, Fig. 8-9c), we can potentially differentiate between the biological sources of the long chain diols, giving us an insight in the phytoplankton community structure between the early-middle Pleistocene. Diatoms from the genus Proboscia has
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been used a marker for upwelling (Smith, 2001; Koning et al., 2001), this because P. alata and P.indica was dominant during an early upwelling.
Sinninghe Damst? et al., 2003; Rampen et al 2007 have also shown that Proboscia diatoms produce long chain, 1,14-diols (C28 1, 14 diol and C30 1, 14 diol ) which are often abundant in sediments from high productivity areas, such as upwelling regions. However Apendinella radian has also been reported to synthesize C28 1, 14 diol and C30 1, 14 diol (Rampen et al., 2011), but it is said to be an important source of long chain diol in estuarine or brackish environments (Rampen et al., 2011). On the other hand eustigmatophytes, algae are known to synthesize C30 1, 15 diol (Volkman et al., 1992; Gelinet al., 1997; Volkman et al., 1999) and provide a source of diols in freshwater sediments (Verteegh et al., 1997). However, only Nannochloropiss has thus far been identified as a marine eustigmatophyte (Volkman et al., 1992; M?janelle et al., 2003) and it is very well possible that long chain diols are produced by different species (Volkman et al., 1992;
Versteegh et al., 1997), which have not yet been identified or cultured. In summary from the alkyl long chain diol profile, we speculate a change in phytoplankton community from eustigmatophyte during the early-Pleistocene to Proboscia in the mid-Pleistocene.
8.7.4. High latitude influence on oceanography of the equatorial Pacific